Propellant compositions have a wide variety of uses, for example, inflation, expulsion, and flotation devices, such as vehicle occupant restraint bags, and commercial and military devices, such as fire suppression devices, piston operated mechanical devices, rocket engines, and munitions. As a result of the diversity and desirability of these compositions, manufacturers strive to improve production methods, reduce costs and waste, and increase safety.
Pyrotechnic propellant compositions typically include a fuel, usually metallic in nature, an oxidizer, and optionally, a binder system that serves as an adhesive, holding the fuel and oxidant in a well-mixed condition. Without a binder, many compositions separate under the influence of gravity or vibration, resulting in performance degradation. In addition, the binder may serve as part of the fuel, and aid in maintaining the final product in a defined physical condition. The binder often causes changes in the burning rate of the composition, so that binder concentrations must be substantially uniform throughout the mass of composition for controllable performance. Therefore, proper mixing and incorporation of the binder during manufacture are key process parameters.
One known method for manufacturing propellant compositions involves dissolving a binder in acetone or other solvent and loading the solution into a muller-type mixer prior to addition of the fuel particles or oxidizer. The concentration of binder in the fluid is typically 10-20%, to keep the viscosity of the fluid down in a convenient working range. Fine metallic powder or other fuel is then added to the mixer, and after a time, an oxidizer, such as polytetrafluoroethylene (PTFE) or a metal salt oxidizer, is also loaded into the mixer. The slurry is mixed until the solvent evaporates to form a dough-like consistency, which is spread on trays and placed in large ovens for complete drying. After drying, the cakes are granulated for feedstock to the process. The process is time consuming and labor intensive. In addition, process workers are exposed to high-hazard conditions.
Another process for manufacturing propellant compositions uses a “shock precipitation” or “Cowles Dissolver” method, as shown in FIG. 1. U.S. Pat. No. 3,876,477 describes a process wherein the binder is dissolved in acetone and placed in a Cowles Dissolver. The fuel and oxidizer components are then suspended in the binder solution and a countersolvent is added while mixing the solution. A large amount (about 4 times the volume of the solution) of countersolvent, e.g., hexane, causes the binder to precipitate from the solvent. As the binder precipitates, the active particles are entrapped in the binder. The solids are then filtered, dried, and pressed or extruded. This process, is also time consuming and results in major waste disposal problems with the large amounts of volatile, flammable solvents used during the process. When performed manually, the operator is also at risk because of the close proximity to the mixing process and the large volume of solvent, as well as the propellant particles. U.S. Pat. No. 6,132,536 also discloses a shock precipitation method, however, the process is automated to reduce safety concerns with the manual process.
Thus, there remains a need for a less-hazardous, less expensive method for making a propellant composition with no reduction of pyrotechnic properties associated with the more hazardous and costly methods currently used. It would be desirable to accelerate production, and avoid the use of large quantities of volatile solvents and the safety hazards associated therewith. The present invention provides a method for manufacturing propellant compositions that reduces the amount of volatile solvent used, accelerates the processing time, and increases process safety.